Method for manufacturing crystalline superfine silk powder

ABSTRACT

The main theme of the invention is to provide a method for manufacturing industrially, by mechanically comminuting silk yarn, crystalline silk fibroin powder below 3 μm in an average particle diameter, which can be used for various applications. A silk substance such as cocoon filament, silk yarn, or raw silk is brought into an alkali aqueous solution under a pressure of 1 through 5 atmospheric pressure at temperatures from 100° C. through 150° C. to reduce the tensile strength of the silk substance to around 0.02 g/d or less. Thereafter, the resultant silk substance is subjected to dealkalization and drying. Subsequently, the resultant dried silk substance is comminuted into powder below 3 μm in an average particle diameter. Thus, the crystalline silk fibroin powder below 3 μm in an average particle diameter is manufactured.

TECHNICAL FIELD

The present invention relates to a method for manufacturing silk fibroinpowder below 3 μm in an average particle diameter from silk substances.

BACKGROUND ART

Silk powder is used as an additive for food, beverages, cosmetics, andpaints. Conventionally known methods for manufacturing silk powder,particularly, crystalline silk fibroin powder, include:

1) a mechanical method for comminuting silk yarn into powder, and 2) achemical method in which, for example, a silk substance is dissolved inan aqueous solution containing an acid or neutral salt or the like;thereafter salt produced by neutralization is removed to form a fibroinaqueous solution; a precipitant is added into this aqueous solution toallow silk fibroin to be precipitated; and then this precipitate isdried by separation or the aforementioned aqueous solution is dried byfreezing or by spraying.

As an example of powdering according to the aforementioned mechanicalmethod, an example for obtaining 300-mesh silk powder (approximately 30through 50 μm in an average particle diameter) for use in cosmetics(Japanese Patent Publication No. Sho 27-3650) has been formerly reportedand an example of obtaining silk fibroin superfine powder 3.25 μm in anaverage particle diameter for use in paints (Japanese Laid-Open PatentPublication No. Hei 6-339924) has been reported.

However, the chemical method produces powder of a low crystalline leveland thus is not available for producing crystalline silk fibroin powderhaving a property intrinsically provided in silk.

Silk substances have a property in that moisture absorbency and waterabsorbency increase as crystallinity becomes lower, so that silk absorbsmoisture or water to become softened and easily transformed.

If silk powder having such low crystallinity is dried after absorptionof moisture or water, the powder tightly coagulates to form very hardlumps and thus provides no value for use. Therefore, in order to holdthe property intrinsically provided in silk, the powder is required tobe a crystalline powder in which the structure intrinsically provided insilk yarn remains. Therefore, it can be said that the mechanicalcomminuting method is desirably employed as an industrial manufacturingmethod of crystalline powder. On the other hand, as described above, theprior art is limited to obtaining powder approximately 3 μm in anaverage particle diameter even by the mechanical comminuting method.

That is, various types of mechanical pulverizers provide a limited sizeof particles by comminuting depending on the property of a raw materialto be pulverized so long as the same type of pulverizers are used,whereby powder with a limitlessly small particle diameter cannot beobtained.

In general, pulverizers employing a higher impact speed provide powderhaving a smaller diameter, however, smaller powder diameters providelower probability of collision of particles. This provides lowerprobability for particles to be applied with stress beyond the fracturespeed, so that the energy efficiency of the pulverizer decreasesacceleratedly with the diameter of particles equal to or less than aspecific value.

Therefore, approximately 3 μm is thought to be the limit in an averageparticle diameter provided by means of the prior art method forobtaining fine powder by comminuting silk yarn.

Concerning the application of silk substances, the present inventorsformerly made it clear that silk substances match skin whereby thepowder thereof is useful as an epidermal cell growth activating rawmaterial (Japanese Patent Application No. Hei 9-333560). In addition,the fine powder of silk substances has also been expected to be usefulas a raw material filler, a coating material, a raw material forcosmetics, etc. The present inventors formerly developed a method forobtaining crystalline silk fibroin powder 3 through 6 μm in an averageparticle diameter by deteriorating a silk substance in strength throughthe exposure thereof to an aqueous solution of an alkali meal compoundunder atmospheric pressure at temperatures of 95° C. or higher, andthereafter by performing dealkalization and drying of the resultant silksubstance, and then by comminuting (U.S. Pat. No.2,615,440). Thereafter,intensive research lead us to a result in that crystalline silk fibroinpowder, the powder being a superfine powder below 3 μm in an averageparticle diameter with the structure intrinsically provided in silk,exerts a property unique thereto so as to be useful for variousapplications.

That is, such findings have been obtained in that the crystalline silkfibroin powder which is a superfine powder below 3 μm in an averageparticle diameter provides excellent formability, improved adhesiveproperty to the skin or the like, improved extensibility, and animproved sense of touch and the like, and is outstandingly suitable forapplications to raw materials for use in cosmetics such as lip sticks,eyebrow paints, hair dyes, eyeliners, powder blushes, and foundations,and for applications to ink additives, resin composite raw materials,and raw materials for paints.

The present invention has been developed in view of the technicalbackground mentioned above. Its object is to provide a method formanufacturing industrially, by mechanically comminuting silk yarn,crystalline silk fibroin powder below 3 μm in an average particlediameter, which can be used for each of the aforementioned applications.

DISCLOSURE OF INVENTION

In order to achieve the aforementioned object, the present inventionemploys technical items as shown below. That is, the present inventionlies in (1) a method for manufacturing crystalline silk fibroin powder,in which the structure of silk yarn remains, below 3 μm in an averageparticle diameter, which is obtained by bringing a silk substance suchas cocoon filaments, silk yarn, and raw silk into contact with an alkaliaqueous solution under a pressure above 1 atmospheric pressure to reducethe strength of the silk substance, then by dealkalizing and drying thesilk substance after that, and by comminuting the resultant silksubstance thereafter.

Furthermore, the present invention lies in (2) a method formanufacturing crystalline silk fibroin powder below 3 μm in an averageparticle diameter by bringing a silk substance such as cocoon filaments,silk yarn, and raw silk into contact with an alkali aqueous solutionunder a pressure of 1 through 5 atmospheric pressure at a temperature of100° C. through 150° C. to reduce the tensile strength of the silksubstance to around 0.02 g/d or less, then dealkalizing and drying theresultant silk substance after that, and by comminuting the driedresultant silk substance thereafter.

Still furthermore, the present invention lies in (3) the method formanufacturing crystalline silk fibroin powder below 3 μm in an averageparticle diameter according to the aforementioned (2) in whichalkalinity of said alkali aqueous solution is pH 9 through pH 12.5.

Still furthermore, the present invention lies in (4) the method formanufacturing crystalline silk fibroin powder below 3 μm in an averageparticle diameter according to the aforementioned (2) in which impactand frictional comminution are combined in order to comminute said driedsilk substance.

Still furthermore, the present invention lies in (5) the method formanufacturing crystalline silk fibroin powder below 3 μm in an averageparticle diameter according to the aforementioned (2) in which impactand frictional comminution are combined in order to comminute said driedsilk substance whereby particle sizes are classified.

Still furthermore, the present invention lies in (6) the method formanufacturing crystalline silk fibroin powder below 3 μm in an averageparticle diameter according to the aforementioned (4) in which in orderto perform comminution by combining impact and frictional comminutions;impact comminution—frictional comminution, frictional-comminution—impactcomminution, or frictional comminution—impact comminution—frictionalcomminution is performed first in sequence to produce silk powder 4through 15 μm in an average particle diameter, and then impactcomminution is performed to produce superfine powder below 3 μm in anaverage particle diameter.

The present invention may be employed, so long as it follows the objectsdescribed in the specification, in a configuration with a combination oftwo or more selected from the group consisting of the aforementioned 1,3, 4, 5, and 6, or in a configuration with a combination of two or moreselected from the group consisting of the aforementioned 2, 3, 4, 5, and6.

According to the present invention, in order to obtain crystalline silkfibroin powder below 3 μm in an average particle diameter, the followingconfiguration is employed. That is,

1) a raw material of a silk substance such as cocoon filaments, silkyarn, and raw silk

2) an alkali aqueous solution is brought into contact

3) a temperature of 100° C. through 150° C.

4) under a pressure of 1 through 5 atmospheric pressure

5) a configuration that reduces the tensile strength of the silksubstance to around 0.02 g/d or less,

6) thereafter, the resultant silk substance is dealkalized and dried,and

7) then, the dried resultant silk substance is comminuted.

Thus, the above configuration provides crystalline silk fibroin powderbelow 3 μm in an average particle diameter. In the present invention, itis important to treat the silk substance so that the tensile strengththereof is reduced to around 0.02 g/d or less. It is difficult to obtainpowder below 3 μm in an average particle diameter with a tensilestrength above 0.02 g/d.

Moreover, it is important to treat the silk substance at a temperatureof 100° C. through 150° C. and under a pressure so as to provide uniformdegradation in strength.

Moreover, it is desirable to employ, in the comminuting process, amulti-stage comminuting method with two stages or more in which theimpact comminution and frictional comminution are combined together.

The silk substance for use in the present invention includes cocoonfilaments, raw silk, silk yarn (yarn from which sericin is excluded bydegumming), and leftover lint thereof.

Moreover, the silk substance can be applied to woven fabrics, knitfabrics, non-woven fabrics, net yarn, and the like, which are formed ofthese respective yarns.

In order to implement the present invention, the silk substance is firsttreated by being brought into contact with an alkali aqueous solution ata temperature above 100° C. preferably, at a temperature within a rangeof 120° C.±10° C. under a pressure.

As an alkaline substance in the alkaline aqueous solution, sodiumcarbonate, sodium hydrogen-carbonate, potassium carbonate, sodiumhydroxide, potassium hydroxide may be used individually or mixed foruse.

The degree of the alkaline aqueous solution (alkalinity) is a pH of 9through 12.5, preferably, a pH of 10.5 through 12.0.

With a pH less than 9, the strength cannot be reduced uniformly withefficiency, while with a pH above 12.5, an excessive dissolved portionof the silk yarn or the like reduces the rate of collection.

Sodium carbonate, an alkaline substance, has a buffering action as anaqueous solution and is very useful since an increase in concentrationhardly causes it to increase the pH thereof to more than around 12.5.

In addition, a slight mixture amount of sodium hydroxide into sodiumcarbonate may cause the total amount of the alkaline substance used todecrease.

The silk substance in the alkali aqueous solution is treated by soakingthe silk substance in the alkaline aqueous solution of a temperatureabove 100° C.

The time for the silk substance to be in contact with the alkalineaqueous solution (Alkali treatment time) is the time necessary to allowthe strength of the silk substance to decrease to a degree suitable forforming superfine powder regardless of whether Bombyx mori silk or wildsilks. (Philosamia cynthia ricini, Antheraea yamamai, Antheraea pernyi,Antheraea mylitta and so on.)

For example, a silk substance is kept in contact with the alkalineaqueous solution until the tensile strength thereof becomes around 0. 02g/d or less and more preferably, up to such an extent that the strengthof the silk yarn is substantially immeasurable in the tensile test (0.01g/d or less), that is, until the silk yarn loses the form thereofavailable for measurement.

In general, the treatment time is 0.5 through 5 hours. In cases wherecocoon filaments, raw silk yarn, or silk yarn has yarn fabrics large indiameter or has sericin adhesion found therewith or in cases where thestrength cannot be readily decreased such as in the case of wild silkyarns, two or three more hours need to be added or the concentration ofthe alkaline substance needs to be increased.

A specific time required for alkali treatment can be determined byexamining the relationship between the easiness of comminution of thesilk substance whose strength has been reduced through the alkalitreatment, the time for the contact with the alkali, and thetemperature.

What is important in reducing the strength of the silk yarn through thealkali treatment is to allow the strength of the silk yarn to decreaseuniformly.

In this case, what is particularly essential is that if the silksubstance to be subjected to the alkali treatment is well separated downto cocoon filaments or silk yarn, powder below 3 μm in an averageparticle diameter may be obtained even with boiling (at the atmospherictemperature) at an alkali treatment temperature of around 100° C.,however, this will result in a variation in strength and lead to aprolonged time in alkali treatment, requiring a greater amount ofalkaline substance and thus providing a significantly reduced ratio ofcollection.

Therefore, it is important to reduce variations in strength after thealkali treatment which depend on the portion of the silk substance suchas cocoon filaments and silk yarn.

The present invention overcomes such a problem by performing the alkalitreatment under a pressure above the atmospheric pressure.

A pressure above the atmospheric pressure can be obtained by putting thesilk substance, an alkali substance, and water into a sealed containerand then by increasing the temperature thereof to a temperature greaterthan the boiling temperature. The pressure ranges substantially from 1through 5 atmospheric pressure. A pressure ranging from 1 through 3atmospheric pressure may be preferably employed in practice particularlyin industrial production.

For example, the alkali treatment for the silk substance would becarried out more uniformly under a pressure of 0.01 atmospheric pressureadded to the atmospheric pressure than under the atmospheric pressureand provide a finer average particle diameter. However, it is far moreeffective to preferably carry out the alkali treatment under more than0.1 atmospheric pressure added to the atmospheric pressure and, morepreferably, under around two atmosphere pressure, at a temperature of120° C.±10° C.

The silk substance after having been subjected to the alkali treatmentis separated from the alkaline aqueous solution, thereafter alkalinesubstances adhered thereto are removed by washing in water to bedealkalized, and then the silk substance is dried.

Neutralization with acids such as hydrochloric acid or tartaric acid canbe employed for dealkalization, and are effective for dealkalization andappropriate for skin.

Moreover, addition of natural acids (such as a liquid squeezed out of acitrus, for example, lemon) after neutralization and washing in waterallows the silk substance to effectively fit to skin and thus makes thesubstance more suitable for a powder raw material for use in cosmetics.

Drying is preferably carried out using a dryer for positive drying,however, natural drying should also be possible.

The superfine powdering of the silk substance is carried out by means ofcombining impact comminution (coarse comminution and superfinecomminution) with frictional comminution.

For example, any of the multi-stage comminuting methods in a) through c)below are employed.

a) impact comminution (coarse comminution)→frictional comminution→impactcomminution (superfine comminution)→(particle size classification)

b) frictional comminution→impact comminution (coarse comminution)→impactcomminution (superfine comminution)→(particle size classification)

c) frictional comminution→impact comminution (coarsecomminution)→frictional comminution→impact comminution (superfinecomminution)→(particle size classification)

Particles are preferably comminuted into 4 through 15 μm in an averageparticle diameter before the last impact comminution (superfinecomminution) is carried out.

Within this range of particle diameter, the impact comminution(superfine comminution) is carried out with great efficiency.

At a result, crystalline silk superfine comminuted particles below 3 μmin an average particle diameter are obtained by the superfinecomminution.

In particular, powder below approximately 1 μm or less can be obtainedby the classification of particle sizes of the resultant silk superfinecomminuted particles.

The resultant silk superfine comminuted particles provide an excellentsense of touch and are ready for forming.

In particular, powder having a diameter of around 1 μm providessignificantly-improved adhesion to skin and extensibility and serves asa skin protective material including raw materials for cosmetics andresin composites. In cases where the material silk substance is broughtinto contact with the alkali aqueous solution to be treated under apressure at a temperature of 100° C. through 150° C., the treatment iscarried out by soaking the silk substance into an alkaline aqueoussolution in a glass container or in a pressure-resistant metalliccontainer made of stainless steel.

At this time, in order to reduce the strength of the material silksubstance uniformly, fiber-shaped silk substances such as cocoonfilaments, raw silk, or silk yarn are preferably separated as much aspossible.

Dealkalization after the alkali treatment is carried out by means ofrepeating washing and dehydrating for neutralization or by theneutralization with acids such as hydrochloric acid or tartaric acid,and thereafter washing is repeated through washing in water and adehydration process.

In the dehydration process, a cloth-shaped filter, for example, a filterwith mesh so fine as to collect fine silk particles around 0.5 μm in aparticle diameter is used.

In the alkali treatment process, non-crystalline portions of thematerial silk substance dissolve gradually into the alkali aqueoussolution and the dissolved silk substance is removed by water at thetime of dehydration. Accordingly, the silk substance to be comminuted iscomprised of a crystalline silk substance in which the structureprovided intrinsically for silk yarn remains (a β-type with fibroinmolecules uniaxially oriented).

In the alkali treatment under a pressure according to the presentinvention, the amount of an alkaline substance required is much lessthan that required in the case of the aforementioned U.S. Pat. No.2,615,440 in which the alkali treatment is carried out under a normalpressure. For example, the ratio of silk to carbonic acid soda must be1:1 in the case of U.S. Pat. No. 2,615,440, while the ratio of silk tocarbonic acid soda is only 1:0.5 in the case of the present invention inwhich the alkali treatment is carried out under a pressure at atemperature of 120° C.

This means that an improved effect is provided, in industrial productionof crystalline superfine silk powder, whereby the number of times ofwashing in water and dehydration after the alkali treatment and theamount of acid required can be naturally reduced.

According to the present invention, silk fibroin superfine powder below3 μm in an average particle diameter can be manufactured from a silksubstance inexpensively with efficiency.

The silk fibroin superfine powder obtained by the present invention iscrystalline and has a crystalline form of a β-type with fibroinmolecules uniaxially oriented in the same manner as the silk yarn asdescribed above. The powder is insoluble in water and has advantageousproperties equivalent to those of silk yarn in hygroscopic property,desorptive property, moisture permeability, etc.

In particular, the powder is readily formable because of fine particlesthereof, providing an improved adhesion and extensibility to the skin,and providing an extremely improved sense of touch, etc. The powder istherefore very useful for applications of raw materials for cosmeticssuch as lip sticks, eyebrow paints, hair dyes, eyeliners, powderblushes, and foundations, and for applications to ink additives, resincomposite raw materials, and raw materials for paints.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a distribution of particle sizes of superfinepowder 0.9 μm in an average particle diameter in the examples.

FIG. 2 is a flow diagram showing a comminuting process after an alkalitreatment.

FIG. 3 is a flow diagram showing a comminuting process after an alkalitreatment.

FIG. 4 is a flow diagram showing a comminuting process after an alkalitreatment.

FIG. 5 is a view showing a rupture test of a supported beam employingpowder as a raw material.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

A wild silkworm (Eri silkworm, Philosamia cynthia ricini) cocoon shellis boiled in a 0.5% sodium carbonate aqueous solution (bath ratio of 50times) to remove sericin, and is then washed in water and dried to formwild silkworm silk yarn.

This silk yarn (fibroin fibers) with the composition shown in Table 1 isput into a stainless steel container and is treated at a temperature of120° C. (under a pressure of 2.02 atmospheres) with the containertightly sealed.

Sodium hydrosulfite is a bleaching agent and Clewat is a trade name of asequestering agent for metals (made by The Teikoku Chemical IndustryCo., Ltd.).

These agents exert an effect on the whiteness of wild silkworm yarnafter the alkali treatment, which are required depending on the materialof the container used and do not particularly exert an effect on thepowdering when using the agents.

The amount of sodium carbonate and the time of alkali treatment exert aneffect on the powdering.

In the case of Table 1, the strength of the silk substance was reducedto around 0.01 g/d.

The alkali treatment method is shown in Table 1.

TABLE 1 Eri silk yarn 20 (g) Sodium carbonate 8 (g) Sequestering agent(Clewat) 5 (g) Sodium hydrosulfite 1 (g) Water 600 (g)

The tensile strength was measured by means of a Tensilon UTM-II.

An alkali solution containing alkali-treated wild silkworm silk yarn wasreduced to 8.5 in pH with hydrochloric acid, then neutralized withtartaric acid, then put into a bag-shaped container of woven fabricswith a high degree of mesh, repeatedly washed in water and dehydratedfour times, and then dried at around 40° C.

In the comminuting treatment, an agitating crusher (a type by Ishikawa)was used for frictional comminution and then a rotary impact pulverizer(Sample Mill KI·-1 made by Fuji Denki Kogyo) was used for powdering toobtain fine silk powder around 12 μm in an average particle diameter.

Moreover, this fine powder is crushed by means of an air-jet pulverizer(Current Jet CJ-10 made by Nisshin Flour Milling Co., Ltd.) andthereafter classified by means of a classifier (Turbo classifier TC-16Nmade by Nisshin Flour Milling Co., Ltd.). Thus, superfine powder wasobtained which has average particle diameters of around 2.6 μm and 0.9μm (FIG. 1).

As can be seen clearly in FIG. 1, particles of 5 μm or more in diameterare contained among particles around 1 μm or less in an average particlediameter that are obtained through the frictional comminution andclassification processes.

This superfine powder can be combined into fabrics around 10 μm in thediameter of fiber to improve the property of the fabrics.

COMPARATIVE EXAMPLE 1

A wild silkworm (Eri silkworm) cocoon shell is boiled in a 0.5% sodiumcarbonate aqueous solution (bath ratio of 50 times) to remove sericin,and is then washed in water and dried to form wild silkworm silk yarn.This silk yarn (fibroin fibers) with the composition shown in Table 2 isthen put into a stainless steel container and then boiled at a normalpressure.

TABLE 2 Silk yarn (Bombyx mori) 20 (g) Sodium carbonate 8 (g)Sequestering agent (Clewat) 5 (g) Sodium hydrosulfite 1 (g) Water 600(g)

The strength of the resultant silk substance was reduced to only 0.06g/d.

The solution containing alkali-treated wild silkworm silk yarn wasneutralized with hydrochloric acid, and then the silk yarn is put into abag-shaped container of woven fabrics with a high density, repeatedlywashed in water and dehydrated four times, and then dried at around 40°C.

For comminuting treatment, an agitating crusher (a type by Ishikawa) wasused for frictional comminution and then a rotary impact pulverizer(Sample Mill KI·-1 made by Fuji Denki Kogyo) was used for powdering.Thereafter, this fine powder is crushed by means of an air-jetpulverizer (Current Jet CJ-10 made by Nisshin Flour Milling Co., Ltd.).Finally, particles had an average particle diameter of 5.7 μm.

COMPARATIVE EXAMPLE 2

A crystalline superfine silk powder was observed by means of apolarizing microscope.

The object of the present invention is to allow the structure providedintrinsically in silk to remain in the crystalline superfine silkpowder. The structure of silk yarn is apt to be destroyed in themechanical comminution of silk yarn, however, the silk powder particleshaving a structure provided intrinsically in silk indicatesbirefringence like silk yarn. Observation of the particles under apolarizing microscope using an inspection plate shows particles inyellow or blue. In contrast, a non-crystalline powder that iscrystallized with alcohol shows no birefringence.

Appearance of birefringence in the non-crystalline powder did not showsuch a high value as that of silk yarn, so that observation by acombination of the shape of particles and birefringence substantiallyshows whether the structure provided intrinsically in silk remains inthe powder. Accordingly, the crystalline superfine silk powder,according to the present invention, was observed under a polarizingmicroscope using an inspection plate. As a result, the observationshowed the same birefringence in 90% or more of particles 1.2 μm in anaverage particle diameter as that found in silk yarn.

On the other hand, silk powder was manufactured by the method for silkfibroin superfine powder described in Japanese Laid-Open PatentPublication No. Hei 6-339924.

As a result, the average particle diameter was 5.2 μm.

Observation of this silk powder under the polarizing microscope showedthe same birefringence as that found in silk in 60 through 70% of theparticles.

The present invention allows the structure provided intrinsically insilk to remain in the particles up to 1 μm in particle diameter.

EXAMPLE 2

As a material substance, bourette (by-silk-yarn a few centimeters orless in the fiber length of silk yarn, Bombyx mori) was used which comesout in a silk spinning process and the bourette having the compositionshown in Table 3 was put into a glass bottle to be treated for two hoursat a temperature of 125° C. (under a pressure of 2.37 atmospheres) withthe bottle tightly sealed.

TABLE 3 Silk yarn (Bombyx mori) 20 (g) Sodium carbonate 4.5 (g)Sequestering agent (Clewat) 4 (g) Sodium hydrosulfite 2 (g) Water 400(g)

The treated silk substance was neutralized, washed in water, and dried,and thereafter comminuted.

The substance was neutralized with hydrochloric acid to have a pH of7±1, and then dehydrated and washed in water repeatedly four times, andfinally dried at a temperature of 40° C.

The comminuting treatment was carried out by means of the same agitatingcrusher used in Example 1.

That is, in the comminuting, the agitating crusher (a type by Ishikawa)was used for frictional comminution; then the rotary impact pulverizer(Sample Mill KI·-1 made by Fuji Denki Kogyo) was used for powdering; andthen the agitating crusher (a type by Ishikawa) was used again forfrictional comminution to obtain fine silk powder of around 11 μm in anaverage particle diameter.

This fine powder was crushed by means of an air-jet pulverizer (CurrentJet CJ-10 made by Nisshin Flour Milling Co., Ltd.) to obtain powder ofaround 2.0 μm in an average particle diameter after the air-jetcomminuting treatment.

The powder was classified thereafter to obtain crystalline superfinesilk powder which have average particle diameters of around 2.5 μm and0.9 μm.

FIG. 2 shows the comminuting process by a flow diagram.

EXAMPLE 3

Raw silk of silkworms, Bombyx mori, was boiled for one hour fordegumming in a 0.1% sodium carbonate aqueous solution 50 times greaterin quantity than the raw silk to form fibroin fabrics (silk yarn).

This silk yarn was used as a material substance to perform alkalitreatment under conditions shown in Table 4.

For the comminuting process after the alkali treatment, case (1), case(2), and case (3) of Table 4 were carried out like in the processesshown in FIG. 2 excluding the classifying process; case (4) of Table 4was carried out as in the process shown in FIG. 3; and case (5) of Table4 as in the process shown in FIG. 4.

FIG. 3 and FIG. 4 show flow diagrams of the comminuting process.

Table 4 shows the average particle diameter of the resultant powder andthe ratio of collection of powder.

In addition, classification of the resultant powder obtained in case (3)of Table 4 provided powder 1.2 μm and 2.5 μm in an average particlediameter.

TABLE 4 Condition (1) (2) (3) (4) (5) Treatment temperature 100 110 120120 120 (° C.) Atmospheric pressure 1.03 1.46 2.02 2.02 2.02 (atm)Treatment time (h) 8 5 2.5 2.5 2.5 Sodium carbonate (g) 20 10 4 4 4 Silkyarn, Bombyx mori 20 20 20 20 20 (g) Sodium hydrosulfite (g) 1 1 1 1 1Water (g) 500 500 500 500 500 Average particle 2.8 2.1 1.8 2.3 3.2diameter (μm) Ratio of collection of 50 55 60 60 60 powder (%)

EXAMPLE 4

Table 5 shows the relationship between the average particle diameter ofthe silk powder particles and the adhesion property thereof, concerningthe silk powder of silkworms, Bombyx mori, which was obtained by themethods of Examples 2 and 3, and in the intermittent processes thereof.

An amount of around 10 g of silk powder with different average particlediameters is placed on a sheet of paper and is spread to about 50 cm².Then, a test piece is placed on the powder to be adhered thereto andfurther, the test piece was covered with an amount of 10 g of the powderof the same average particle diameters.

Subsequently, the test piece buried in the powder was lifted verticallywith tweezers. Then, the test piece was weighed with powder adheredthereto to calculate the amount of powder adhered to the surface and thereverse of the test piece per unit area.

The measurement was carried out in a room at a temperature of 20° C.with 65% HR.

The test piece had the shape of a film with a side thereof having anarea of around 10 cm² (10 cm²±0.5 cm²).

Three types of materials of metal (aluminum), resin (polyethylene), andnatural substance (silk fibroin) were used in the test piece.

As can be seen clearly in Table 5, over a range of average particlediameters around 3 μm or less, the smaller the particles diameter, thelarger the amount of powder adhered to the test piece becomes in anymaterial.

TABLE 5 Relationship between average particle diameter and adhesionproperty of silk powder particles Amount of powder adhered to the testpiece Average particle (mg/cm²) diameter (μm) Silk fibroin PolyethyleneAluminum 0.9 3.003 0.863 1.418 1.21 2.887 0.761 1.233 2.32 1.390 0.5891.045 2.57 1.148 0.520 0.904 3.65 0.613 0.311 0.566 5.40 0.452 0.2970.491 12.20 0.304 0.206 0.281

EXAMPLE 6

Using silk powder obtained by the methods of Examples 1 through 3, thesense of touch thereof was evaluated in a panel experiment by fivewomen.

The test was carried out in a room of constant temperature and constanthumidity at 20° C. with 65% RH. An amount of 1 g of silk powder wasplaced on the inside of the forearm of one arm and was pressed with theother hand to be rubbed in various directions. The subjects were askedto fill in a questionnaire of how they felt at that time. The resultsare shown in Table 6.

As shown in Table 6, it can be seen that improved adhesion to the skin,extensibility, and smoothness are provided by the silk powder belowaround 3 μm in an average particle diameter, particularly by that below1μm in diameter.

TABLE 6 Particle diameter (μm) Sense of touch 12 Rough feeling 8 Notrough to touch 5 Soft to touch 3.7 Soft to touch 2.3 Very soft to touch,good in adhesion to the skin, and good in extensibility on the skin 0.9Very soft to touch, very good in adhesion to the skin, and very good inextensibility on the skin

EXAMPLE 7

In order to examine the formability of powders such as Talc (JA-46R),Mica (No.5500), Titan (A-100), and Kaolin (JP-100), rupture tests wereconducted on supported beams with the powders employed as raw materials.

In the tests, as shown in FIG. 4, a beam was supported at an interval of4 cm and loaded at point A. A magnitude of the load provided at the timeof rupture was measured with Tensilon UTM-II.

The silk powder used was the same as that shown in Table 4 in Example 4.An amount of 4.0 g of each of the powders was put into a mold of 10 mm(W)×100 mm (L)×30 mm (H) and subjected to a load of 30 kg/cm² to beformed.

The results are shown in Table 7.

Concerning powders for cosmetics such as Talc, Mica, Titan, and Kaolin,these powders (4.0 g) were put into a mold with 10 mm (W)×100 mm (L)×30mm (H) and subjected to a load of 30 kg/cm² to be formed.

However, these formed powders were ruptured with a load around 1 g orless. The formed shapes were hardly sustained and thus the formabilityof the powders were too inferior to conduct measurement of rupturestrength by such a method.

In order to prevent rupture caused by impacts to cosmetics in the formof powder, in the rupture test shown in FIG. 5, the powder requiresstrength of around 20 g and preferably 30 g or more in strength.Accordingly, an amount of a 4.0 g mixture of 50 wt % of theaforementioned silk powder 1.21 μm in an average particle diameter and50 wt % of Talc (JA-46R) was put into a mold of 10 mm (W)×100 mm (L)×30mm (H) and subjected to a load of 30 kg/cm² to be formed like theforegoing. Then, the rupture load was measured to be 31.2 g. The silkpowder can be used as a molding material.

TABLE 7 Average particle 0.9 1.21 2.32 2.57 3.65 5.40 12.20 diameter(μm) Rupture load (g) 53.8 63.5 36.3 44.8 21.6 12.2 4.3

Industrial Applicability

The crystalline superfine silk powder manufactured according to thepresent invention is excellent in applications to raw materials for usein cosmetics such as lip sticks, eyebrow paints, hair dyes, eyeliners,powder blushes, and foundations, and in applications to ink additives,resin composite raw materials, and raw materials for paints.

What is claimed is:
 1. A method for manufacturing crystalline silkfibroin powder below 3 μm in average particle diameter comprising thesteps of: bringing a silk substance into contact with an alkali aqueoussolution under a pressure exceeding 1 but no more than 5 atmosphericpressures and at a temperature exceeding 100° C. but no more than 150°C. to reduce the tensile strength of the silk substance to around 0.02g/d or less, dealkalizing and drying the resultant silk substance afterthat, and comminuting the dried resultant silk substance thereafter. 2.The method for manufacturing crystalline silk fibroin powder below 3 μmin average particle diameter according to claim 1, wherein said alkaliaqueous solution has a pH of 9 through 12.5.
 3. The method formanufacturing crystalline silk fibroin powder below 3 μm in averageparticle diameter according to claim 1, wherein impact and frictionalcomminution are combined to comminute said dried silk substance.
 4. Themethod for manufacturing crystalline silk fibroin powder below 3 μm inaverage particle diameter according to claim 1, wherein impact andfrictional comminution are combined in order to comminute said driedsilk substance, and further comprising the step of classifying thecomminuted dried silk substance.
 5. The method for manufacturingcrystalline silk fibroin powder below 3 μm in average particle diameteraccording to claim 3, comprising the steps of: in order to performcomminution by combining impact and frictional comminutions, first,performing impact comminution-frictional comminution,frictional-comminution-impact comminution, or frictionalcomminution-impact comminution-frictional comminution in sequence toproduce silk powder of 4 through 15 μm in average particle diameter, andsubsequently, performing impact comminution to produce superfine powderbelow 3 μm in average particle diameter.